Helfrich's concept of intrinsic force and its molecular origin in bilayers and monolayers

Bilayers and monolayers are excellent models of biological membranes. The constituents of the biological membranes such as lipids, cholesterols and proteins are chiral. Chiral molecules are abundant in nature (protein, nucleic acid and lipid). It is obvious that relationship between chirality and morphology (as well as function) of biological membrane is of interest for its fundamental importance and has technological implication regarding various membrane functions. The recent years have witnessed that a number of experimental studies in biomimetic systems have shown fascinating morphologies where chirality of the constituent molecule has decisive influence. Significant progress is made towards the understanding of these systems from the theoretical and computational studies. Helfrich's concept of intrinsic force arising from chirality is a milestone in understanding the biomimetic system such as bilayer and the related concepts, further progresses in molecular understanding made in recent years and experimental studies revealing the influence of chirality on morphology are the focus of the present review. Helfrich's concept of intrinsic force arising due to chirality is useful in understanding two-dimensional bilayers and one-dimensional monolayers and related mimetic systems. Various experimental techniques are used, which can probe the molecular architecture of these mimetic systems at different length scales and both macroscopic (thermodynamic) as well as microscopic (molecular) theories are developed. These studies are aimed to understand the role of chirality in the molecular interaction when the corresponding molecule is present in an aggregate. When one looks into the variety of morphologies exhibited by three-dimensional bilayer and two-dimensional monolayer, the later types of systems are more exotic in the sense that they show more diversity and interesting chiral discrimination. Helfrich's concept of intrinsic force may be considered useful in both cases. The intrinsic force due to chirality is the decisive factor in determining morphology which is explained by molecular approaches. Finally, biological and technological implications of such morphological variations are briefly mentioned.